This invention relates in general to electrophotography and more specifically, to an electrophotographic imaging member and process for forming the imaging member.
In the art of electrophotography an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer.
The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated, duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the ground plane of many modern photoconductive imaging members must be highly flexible and adhere well to supporting substrates, particularly belt type photoreceptors over many thousands of cycles.
One type of ground plane which is gaining increasing popularity for belt type photoreceptors is vacuum deposited aluminum. However, aluminum films are relatively soft and exhibit poor scratch resistance during photoreceptor fabrication processing. In addition, vacuum deposited aluminum exhibits poor optical transmission stability after extended cycling in xerographic imaging systems. This poor optical transmission stability is the result of oxidation of the aluminum ground plane as electric current is passed across the junction between the metal and photoreceptor. The optical transmission degradation is continuous and, for systems utilizing erase lamps on the nonimaging side of the photoconductive web, has necessitated erase intensity adjustment every 20,000 copies over the life of the photoreceptor.
Further, the electrical cyclic stability of an aluminum ground plane in multilayer structured photoreceptors has been found to be unstable when cycled thousands of times. The oxides of aluminum which naturally form on the aluminum metal employed as an electrical blocking layer prevent charge injection during charging of the photoconductive device. If the resistivity of this blocking layer becomes too great, a residual potential will build across the layer as the device is cycled. Since the thickness of the oxide layer on an aluminum ground plane is not stable, the electrical performance characteristics of a composite photoreceptor undergoes changes during electrophotographic cycling. Also, the storage life of many composite photoreceptors utilizing an aluminum ground place can be as brief as one day at high temperatures and humidity due to accelerated oxidation of the metal. The accelerated oxidation of the metal ground plane increases optical transmission, causes copy quality non-uniformity and can ultimately result in loss of electrical grounding capability.
After long-term use in an electrophotographic copying machine, multilayered photoreceptors utilizing the aluminum ground plane have been observed to exhibit a dramatic dark development potential change between the first cycle and second cycle of the machine due to cyclic instability. The magnitude of this effect is dependent upon cyclic age and relatively humidity but may be as large as 350 volts after 50,000 electrical cycles. This effect is related to interaction of the ground plane and photoconductive materials.
Many metals or other materials which are highly oxidatively stable, form a low energy injection barrier to the photoconductive material when utilized as a ground plane in a photoconductive device. A hole blocking layer will not form on these oxidatively stable layers thus rendering these devices non-functional as photoconductive devices.
Thus, there is a continuing need for photoreceptors having ground planes that exhibit improved scratch resistance, greater optical transmission stability, extended electrical cyclic stability, adequate injection barrier characteristics, longer storage life at high temperatures and humidity and stable dark development potential characteristics.